Signal processing circuits used, for example, in an automative application typically require a temperature sensing feature to ensure its accuracy. Typically, signal processing circuits have temperature dependent variables that affect the accuracy of the output signal of the circuit. In fact, the accuracy of the output signal can be affected over the entire range of temperature that the processing circuit is operated. The accuracy of the output voltage is particularly important for automative applications as the signal for processing circuits are typically operated at an elevated temperature.
One commonly used method for sensing the temperature variation and digitize in a signal processing circuit is to compare a proportional to absolute temperature (V.sub.ptat) voltage with a reference (V.sub.ref) voltage that is insensitive to temperature using an analog to digital converter (A/D). Dividing the V.sub.ptat voltage with the V.sub.ref voltage generates a ratio, which is proportional to the temperature. This ratio can be in the form of a digital pulse stream, if a sigma delta modulator is used as an A/D. A digital filter can convert this to a digital word.
A bandgap circuit is conventionally used to provide the reference voltage to the processing circuit, since this the bandgap circuit is insensitive to temperature variations. However, the bandgap reference voltage is also insensitive to power supply voltage variation since the bandgap voltage is a physical constant. Thus, the temperature dependent output generated by the V.sub.ptat and V.sub.ref voltages will be independent of the power supply voltage.
A signal conditioning circuit usually requires a ratiometric output voltage over a wide temperature range. When the power supply fluctuates within the specified range, the output voltage of the circuit must vary over the whole temperature variation. The output voltage of the circuit must vary in a specified range. During the calibration a process correction at room temperature of the signal is fairly straightforward. The output signal typically remains ratiometric to the supply voltage at room temperature. However, the temperature calibration portion that is also ratiometric to supply is not as straightforward. If the V.sub.ptat and V.sub.ref voltages are used directly to correct the signal temperature related parameters for a fixed supply voltage, the output is fine for that voltage. However, as supply voltage varies the output error will increase beyond the narrow range over temperature due to independence of V.sub.ptat and V.sub.ref of the supply voltage. Accordingly, what is needed is a system and method for providing an output voltage which is both temperature dependent and also ratiometric to the supply voltage to replace V.sub.put and V.sub.ref. The present invention addresses such a need.